The vertebrate spine is made of bony structures called vertebral bodies that are separated by soft tissue structures called intervertebral discs. The intervertebral disc is commonly referred to as a spinal disc. The spinal disc serves as a mechanical cushion and a connection between the vertebral bodies, permitting controlled motions between vertebral segments of the axial skeleton. The disc acts as a synchondral joint and allows physiologic degrees of flexion, extension, lateral bending, and axial rotation of adjacent vertebral bodies relative to one another. The disc must allow these motions and yet must have sufficient elastic strength to resist the external forces and torsional moments caused by the vertebral bodies.
The normal disc is a mixed avascular structure comprising two vertebral end plates (“end plates”), an annulus fibrosis (“annulus”) and a nucleus pulposus (“nucleus”). The end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone. The end plates act to attach the disc to the spongy cancellous bone of the vertebral bodies.
The annulus of the disc is a tough, outer fibrous ring about 10 to 15 millimeters in height and about 15 to 20 millimeters in radial thickness. The structure of the annulus is somewhat like an automobile tire, with 15 to 20 overlapping plies. Its fibers extend generally helically and are inserted into the superior and inferior vertebral bodies at a roughly 30-40 degree angle to the central axis of the spine in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies of the annulus are less firmly attached to each other. The attached fibers also prevent the disc from extruding laterally as a consequence of the complex twisting motion of the spine.
Inside the annulus is a gel-like nucleus with high water content. The nucleus acts as a liquid to equalize pressures within the annulus. The material consistency and shape of a normal nucleus pulposis is similar to the inside of a jelly doughnut. The loose fluid-like nature allows the nucleus to shrink with compressive forces or swell from osmotic pressure. The ion concentration of the nucleus can create an osmotic swelling pressure of about 0.1 to about 0.3 MPa. As a result, the gel-like nucleus can support an applied load yet can be compressed or temporarily deformed to a limited extent. Together, the annulus and nucleus support the spine while flexing, extending, compressing, or rotating in response to forces produced by the adjacent vertebral bodies during bending, lifting, etc.
The compressive load on the disc changes with posture. For example, when the human body is supine, the compressive load on the third lumbar disc is about 300 Newtons (N), which rises to about 700 N when an upright stance is assumed. The compressive load increases yet again, to 1200 N, when the body is bent forward by only 20 degrees. Resultant pressure within the nucleus pulposis of a spinal disc similarly varies between, for example, 0.5 MPa when in a relaxed standing posture, to 2.3 MPa or more when lifting a weight with the back roundly flexed.
A spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the annulus fibers are weakened or torn and the inner material of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal annular confines The mass and/or physiologic reaction of a herniated or “slipped” nucleus tissue can compress a spinal nerve, resulting in leg pain, loss of muscle strength and control, and even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and dehydrates with consequent loss in disc height. Consequently, the volume of the nucleus decreases, causing the annulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping plies of the annulus buckle and separate, either circumferential or radial tears may occur in the annulus, potentially resulting in persistent and disabling back pain. Adjacent, ancillary facet joints between vertebrae bones may also be forced into an overriding position, which may cause additional back pain. The most frequent site of occurrence of a herniated disc is in the lower lumbar region. The cervical spinal discs are also commonly affected.
In the United States, low back pain accounts for the loss of many workdays. Degeneration of an intervertebral disc is one of the most common causes of low back pain and therefore frequently requires treatment. When conservative treatment such as activity modification, medications, physical therapy, or chiropractic manipulation fail, more aggressive measures, such as surgical treatment, may be required. Spinal fusion has been the mainstay of surgical treatment for recalcitrant low back pain secondary to a degenerated disc, but spinal fusion causes stiffness of the vertebral segment and therefore places increased stresses on adjacent vertebral levels. Replacement of the intervertebral disc with a device that maintains the height of the disc while still maintaining compressibility and motion is highly desirable and is likely to decrease the back pain associated with a diseased intervertebral disc.
An early design for an artificial disc was primarily a round stainless steel ball intended to replace the intervertebral disc. This resulted in the steel ball subsiding into the vertebral body and did not maintain disc height nor provide compressibility. Subsequent designs of intervertebral disc replacements incorporated a ball and socket design but used metal end plates to fit adjacent to the vertebral bodies to prevent subsidence. Most disc replacements of this ball and socket type of design do not allow for a mobile center of rotation in both the axial plane and the sagittal plane. Many of these designs also lack any type of resiliently compressible material within the device to absorb compressive forces.
Other designs for artificial disc replacement incorporate some form of compressive springs. This may result in motion of metal on metal where the springs are attached to the endplates. This potentially causes release of metal particulate debris into the tissues, which can stimulate foreign body reaction. Foreign body reactions can result in resorption of adjacent bone and subsequent subsidence, loosening and pain. Other problems with compressive spring-type prostheses are that they do not resist translational forces well and will eventually fatigue. These devices also lack a mobile instantaneous axis of rotation.
Some current disc prostheses include a solid core of an elastomeric material, such as a polyolefin, to act as a compressible core between two metal end plates. Devices of this type present the problem of having to attach a substance of consistent elasticity to a metal end plate. These devices do not resist shear or translational forces well.
What is desired, then, is a prosthetic spinal disc able to allow limited flexion and rotation of vertebra bodies between which the prosthesis is implanted, which is compressible, and which can be implanted using a selected one of several different approach directions.